1. Field of the Invention
The present invention relates to waveguide structures for use in propagation of electromagnetic waves such as microwaves and millimeter waves, and in particular to electromagnetic-band-gap (EBG) structures for suppressing propagation of electromagnetic waves in specific frequency bands. The present invention also relates to printed-circuit boards including waveguide structures.
The present application claims priority on Japanese Patent Application No. 2008-164338, the content of which is incorporated herein by reference.
2. Description of Related Art
Various technologies regarding waveguide structures and printed-circuit boards for suppressing propagation of electromagnetic waves in specific frequency bands have been developed and disclosed in various documents.    Patent Document 1: U.S. Patent Application Publication, US 2005/019051 A1    Patent Document 2: U.S. Patent Application Publication, US 2005/0205292 A1
Recently, methods for artificially controlling frequency dispersions of electromagnetic waves by use of repetitively aligned conductive patches have been provided. Among those structures, structures having band gaps in frequency dispersions are referred to as EBG structures, which are expectedly applied to filters for suppressing propagation of unwanted noise in printed-circuit boards or substrates of device packages.
Patent Document 1 teaches an EBG structure for reducing noise propagation between parallel plates. The EBG structure includes conductive patches, which are aligned in a third layer between parallel plates and which serve as capacitances for one conductive plane of the parallel plates, and admittances (or shunts) for connecting the conductive patches to another conductive plane of the parallel plates, wherein the admittances are repetitively aligned in a one-dimensional manner or a two-dimensional manner along the parallel plates. Due to band gaps occurring in frequency bands in which the admittances serve as inductances in the EBG structure, it is possible to set band gaps by controlling serial-LC resonance frequencies of admittances.
Securing adequate capacitances and inductances in the above EBG structure leads to increasing the areas of conductive patches or increasing the lengths of conductive vias, which in turn makes it difficult to reduce the sizes of structures.
Patent Document 2 teaches a structure in which chip capacitors are mounted on the surface and are connected in parallel between conductive planes and conductive patches. This structure increases capacitances without increasing the areas of conductive patches.
However, Patent Document 2 discloses that using chip capacitors increases the number of parts so as to increase the manufacturing cost.
In the above circumstances, the inventor has recognized that it is necessary to fabricate an EBG structure (or a waveguide structure) and a printed-circuit board with a reduced size, without using chip components, and with low manufacturing cost.